One of X-ray diagnosis apparatuses is a computed tomography (CT) apparatus. The CT apparatus comprises an X-ray tube for radiating X-ray fan beams, and a radiation detector comprising a large number of radiation detection elements. X-ray fan beams radiated from the X-ray tube pass through an object to be measured, and are detected by the radiation detector. The detected data are analyzed by a computer to display a cross section of the object. The measured data are analyzed by a computer to calculate X-ray absorbance at each position in each cross section of the object by a computer, thereby forming an image based on the X-ray absorbance.
As radiation detectors for detecting radiations such as X-rays, etc., radiation detectors comprising radiation-detecting elements obtained by combining ceramic scintillators produced by sintering rare earth oxysulfide powder such as Gd2O2S, Y2O2S, Lu2O2S, etc. comprising Pr, Ce, Eu, Tb, etc. as luminescent elements, and silicon photodiodes have been developed and put into practical use. The radiation detector generally has a structure comprising one or more lines of plural radiation-detecting elements to simultaneously detect X-rays at many positions. In the radiation detector comprising ceramic scintillators, detecting elements can easily be made small to increase the number of channels, thereby obtaining high-resolution image.
In such radiation-detecting elements, when scintillators absorbing radiations emit light with large intensity (luminescence intensity), they have high sensitivity. Diagnosis apparatuses utilizing radiations are recently required strongly to reduce radiations to which humans are exposed. As a result, it has become important to shorten the scanning time. Shorter scanning time from the present level results in shorter integration time in one detecting element, thereby reducing the total amount of radiations absorbed during the integration time. Accordingly, scintillators having high luminescence efficiency (large luminescence intensity) are particularly needed.
Japanese Patent 2989184 discloses a method for producing a ceramic scintillator having sufficient luminescence intensity, comprising the steps of mixing rare earth oxide with sulfur and an alkali flux, calcining the resultant mixture in an alumina crucible, disintegrating the calcined product in pure water, washing the product with pure water, hydrochloric acid and warm water successively to obtain scintillator powder, introducing this powder into a soft iron capsule, and subjecting it to hot-isostatic pressing for sintering.
JP 2000-313619 A discloses a method for inexpensively producing rare earth oxysulfide powder used in scintillators, comprising the steps of dispersing at least one rare earth oxide in water, adding 1 mol of sulfuric acid or sulfate corresponding to at least one rare earth to 1 mol of rare earth oxide, calcining the resultant powdery precipitate, and reducing the resultant rare earth oxysulfate.
JP 2004-525848 A discloses a method for producing a high-density, translucent scintillator ceramic, which comprises wet-pulverizing rare earth oxysulfide in a pulverizing organic liquid to powder having particle sizes of less than 10 μm, forming this powder into a green body having a density of 40-60%, and sintering the green body at a temperature of 1200-1450° C. under atmospheric pressure in vacuum or an inert gas.